Foam sheet, method and apparatus for widening foam sheet

ABSTRACT

An apparatus 1 for widening a foam sheet S obtained immediately after heating and foaming a foam sheet which has not been yet widened, while the foam sheet S is continuously received or supplied. The sheet widening apparatus 1 comprises: a pair of chains 2 constructed in the form of endless chains composed of links 22 having pin parts 21 and a driving device 3 meshing with these chains 2 and driving the chains 2 along such loci as to widen the space between the chains 2 from sheet receiving start points to sheet receving end points. A method for widening a foam sheet by the sheet widening apparatus 1 comprises the steps of piercing both the side parts of the foam sheet S with the pins 21 at the sheet receiving start points; widening the space of width between the pins 21 along gently curved loci by drying the chains 2 while the pins 21 penetrating the foam sheet; and receiving the foam sheet S from the sheet receiving start points to the sheet receiving end points while the foam sheet is widened. The foam sheet S obtained after the widening step in accordance with the sheet widening method.

This is a continuation of application Ser. No. 08/197,971, filed on Feb.8, 1994, which is abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a foam sheet obtained by widening afoam sheet foamed by heating and a method and an apparatus for wideninga foam sheet.

Generally, in case where a foam sheet is heated and softened to formmold goods, the mold goods have been sometimes wrinkled in accordancewith the slackening phenomenon (draw-down) of the foam sheet caused bythe self-weight of the foam sheet, or the thickness of the foam sheethas not been uniform, so that the commercial value of the mold goods isdeteriorated.

Thus, in order to obtain a foam sheet in which a draw-down phenomenonthereof at the time of thermally forming it, a technique has beenconventionally known that a foam sheet obtained immediately after a foamsheet is foamed by heating in the manufacturing step thereof is widened.

As a method for widening a foam sheet obtained immediately after foaminga foam sheet which has not been yet widened, a sheet widening techniquesuch as a technique using a belt-rotary type or disk rotary-type sheetwidening apparatus has been known which is vacuum-attached to both theends of a foam sheet to increase the width of the foam sheet (Forexample, refer to, Japanese Examined Patent Publication No. 48-9955).

As an alternative method for widening a foam sheet, a sheet wideningmethod using a sheet widening apparatus has been known which is adaptedto increase the width of the foam sheet while both the ends of the foamsheet are held by holding parts including a pair of rotary belt likesupporters and a plurality of holding wheels for holding both the endparts of the foam sheet (For example, refer to Japanese UnexaminedPatent Publication No. 2-72938).

However, the above mentioned conventional sheet widening techniques aremethods for increasing the width of a foam sheet merely by a vacuumforce or a holding power, and therefore, the foam sheet cannot be alwaysheld completely. Thus, a problem that the foam sheet is slipped ordisplaced upon widening or the foam sheet cannot be widened with asufficient holding power is liable to arise.

Therefore, the foam sheet to be widened in accordance with thesetechniques can be widened only within the range of the holding power ofthe holding parts, so that a completely widened foam sheet cannot beobtained.

In the case of the sheet widening apparatus using the vacuum-attachingmeans, such a disadvantage arises that the apparatus is complicated orbecomes large in size in order to obtain a satisfactory vacuum force.

Further, also in the case of the sheet widening apparatus using theholding parts for holding a foam sheet, a problem resides in that amechanism becomes complicated due to the rotary belt like supporters orthe plurality of holding wheels and the size of the apparatus is large.Particularly, in the case of the sheet widening technique using theholding parts, when the foam sheet is abruptly widened, the foam sheetcannot be held, and therefore, the foam sheet has been gradually widenedalong gently tapered loci over a long sheet receiving distance. In thiscase, however, only both the side parts of the foam sheet, namely, onlythe parts of the foam sheet held by the holding parts are liable to bewidened and the foam sheet which is evenly widened all over thedirection of width cannot be obtained. In addition, if the foam sheet iswidened over the long sheet receiving distance, the foam sheet is cooledon the way of sheet receiving, so that it is necessary to prepare aspecial heat source for reheating the foam sheet and a temperatureadjustment is difficult.

SUMMARY OF THE INVENTION

The present invention has been made in order to overcome these problems.Accordingly, it is an object of the present invention to provide a foamsheet the width of which is completely and uniformly increased in thedirection of width, and a sheet widening method and a sheet wideningapparatus for simply and easily manufacturing the foam sheet.

For solving the above mentioned problems, a foam sheet of the presentinvention is a kind of foam sheet widened while it is moved with boththe ends thereof fixed by a sheet widening apparatus. This foam sheet isobtained in such a manner that a crosslinked polyolefine foam sheetobtained immediately after a foam sheet which has not been yet widenedis foamed by heating is widened along gently curved loci havinginflection points.

A sheet widening method according to the present invention for solvingthe above mentioned problems comprising the steps of: piercing with pinsboth the side parts of a crosslinked polyolefine foam sheet obtainedimmediately after a foam sheet which has not been yet widened is foamedby heating when the polyolefine foam sheet is received by a sheetwidening apparatus; and widening the space of width between the pinspiercing the foam sheet along gently curved loci, while the foam sheetis received from a sheet receiving start point to a sheet receiving endpoint with the pins penetrating the foam sheet.

Further, a sheet widening apparatus according to the present inventionfor solving the above mentioned problems comprising: a pair of chainsconstructed in an endless chain form composed of links having pins anddriving means meshing with these chains and driving the chains alongsuch loci as to widen the space between these chains.

Still further, for solving the above described problems, a foam sheetwidening apparatus of the present invention as described in claim 3 ischaracterized in that the driving means meshing with the pair of chainsand provided in the sheet receiving start points or the sheet receivingend points are so attached as to be angled relative to the advancingdirection of the foam sheet or the inclination angle thereof can bechanged.

Still further, for solving the above described problems, a foam sheetwidening apparatus of the present invention as described in claim 4 ischaracterized in that the pins piercing the both side parts of the foamsheet are so attached as to be inclined relative to the links formingthe chains.

Furthermore, for solving the above mentioned problems, a foam sheetwidening apparatus of the present invention as described in claim 4 ischaracterized in that the pins piercing the side parts of the foam sheethave larger core angle than the inclination angle of the chains relativeto the advancing direction of the foam sheet.

According to the sheet widening method of the present invention, thespace of width between the pins is widened while both the side parts ofthe foam sheet are pierced with the pins, and therefore, this means thatthe foam sheet is substantially widened by the width between the pinswhich is increased. The space of width between the pins is widened alonggently curved loci having inflection points, so that even if the spaceof width of the pins is abruptly widened, the side end parts of the foamsheet is not broken or do not get out of place. Therefore, a drawingforce is uniformly applied all over the foam sheet.

Further, according to the sheet widening apparatus of the presentinvention, the pin parts of the chains pierce both the side parts of thefoam sheet at the sheet receiving start points, so that they hold boththe side parts of the foam sheet. The driving means drive the chainshaving the pins piercing the foam sheet so that the foam sheet iswidened from the receiving start point to the receiving end point of thefoam sheet.

Still further, according to the sheet widening apparatus of the presentinvention, the driving means engaging with a pair of chains at the sheetreceiving start point are so attached as to be angled relative to theadvancing direction of the foam sheet, or the inclination angle of thedriving means can be changed. Therefore, the initial widening step ofthe foam sheet can be carried out depending on the characteristics ofthe foam sheet.

Also, since the driving means provided at the sheet receiving startpoint and the driving means provided at the sheet receiving end pointare so attached as to be angled relative to the advancing direction ofthe foam sheet or the inclination angle of the driving means can bevaried, the inclination angle of the driving means can be changed basedon the characteristics of the foam sheet, a space between the drivingmeans at the sheet receiving start point and the driving means at thesheet receiving end point can be decreased, which contributes to a smallsize of apparatus.

Still further, according to the sheet widening apparatus of the presentinvention, the pins piercing the side parts of the foam sheet are soattached as to be respectively inclined relative to the links formingthe chains. Therefore even when the advancing direction of the chains isdifferent from that of the foam sheet, the pins can be directed to thesame direction as the advancing direction of the foam sheet at the timeof pulling the pins out of the foam sheet. Thus, the pins can be easilypulled out of the foam sheet and such inconveniences, can be prevented,as the winding of the foam sheet on the chains or the breakage of thefoam sheet due to the difficulty in pulling the pins out of the foamsheet or the like.

Furthemore, according to the sheet widening apparatus of the presentinvention, since the pins piercing the side parts of the foam sheetrespectively have a larger cone angle than the inclination angle of thechains relative to the advancing direction of the foam sheet, theinclination of the pins relative to the advancing direction of the foamsheet is absorbed because of this large cone angle. Therefore, the pinscan be readily pulled out of the foam sheet and various troubles, forexample, the twining of the foam sheet on the chains or tearing of thesheet due to the difficulty to pull the pins out of the foam sheet canbe prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

Other feature and advantages of the present invention will be apparentfrom the following description taken in connection with the accompanyingdrawings, in which

FIG. 1 is a schematic front view showing the entire construction of anapparatus for widening a foam sheet;

FIG. 2 is a longitudinally sectional view taken along a line II--II inFIG. 1;

FIG. 3 shows a partial side view and a sectional view schematicallyillustrating the entire construction of a chain;

FIG. 4 shows a partial side view and a sectional view illustratinganother example of a chain;

FIG. 5 is a schematic view for explaining a sheet widening methodaccording to the present invention;

FIG. 6 is a schematic view for explaining another sheet widening methodaccording to the present invention;

FIG. 7 is a schematic view for explaining still another sheet wideningmethod according to the present invention;

FIG. 8 is a sectional view showing still another example of a chain ofthe sheet widening apparatus according to the present invention;

FIG. 9 is a plane view showing an example of the attaching direction ofa pin part of the sheet widening apparatus according to the presentinvention;

FIG. 10 is a plane view showing another example of the attachingdirection of a pin part of the sheet widening apparatus according to thepresent invention;

FIG. 11 is a schematic view for explaining the state of the chain shownin FIG. 8 when the space between the chains is widened;

FIG. 12 is a schematic view for explaining the state of a chain whenwidening the space between the chains without using the chain shown inFIG. 8; and

FIG. 13 is a sectional view showing a still further example of a chainof the sheet widening apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, an embodiment of the present invention will be describedhereinbelow with reference to the accompanying drawings.

FIGS. 1 and 2 schematically illustrate the entire construction of anapparatus 1 for widening a foam sheet S.

This sheet widening apparatus 1 is designed to continuously widen thefoam sheet S obtained immediately after a foam sheet which has not beenyet widened is foamed by heating while the foam sheet S is continuouslyreceived. The sheet widening apparatus includes chains 2 and a drivingdevice 3 for driving the chains along such loci as to widen a spacebetween the chains from sheet receiving start points to sheet receivingend points.

As shown in FIG. 3, each of the chains is constructed in a form of anendless chain by connecting links 22 having pins 21. Receiving parts 24are extended from link plates 23 of the link 22. To the receiving parts24, a base member 25 with a pin 21 is fixed by means of bolts 26 andnuts 27. While the links 22 are connected in the form of an endlesschain, the pins 21 protrude to the outer surface side of the chain 2.While, in this embodiment, the receiving parts 24 are extended so thatthe two link plates 23 are connected, the receiving part 24 may beextended only from one link plate 23 and the pin 21 may be projected onthe receiving part 24, as shown in FIG. 4. The pin 21 may be so attachedas to be vertically inclined.

The driving device 3 includes a pair of substrates 31 so fixed as to beopposed to each other at both end parts of a slide rail 30. In theinside of the fixed substrates 31, a pair of slide substrates 32 areprovided so as to be opposite to each other. The fixed substrates 31 areconnected to the slide substrates 32 respectively by means of bolts 33having handles 331. Each of the slide substrates 32 can be slid byrotating each of the handles 331. Namely, the width between the slidesubstrates 32 can be adjusted in accordance with the width of the foamsheet S by sliding the slide substrates 32.

In the inside of each of the slide substrate 32, there are provided adrive gear 34, a tension gear 35, an outer gear 36 and an inner gear 37which engage with the chain. The drive gear 34 is rotated by a motor 341attached to the slide substrate 32 to thereby drive the chain 2. Thetension gear 35 is provided on the slide substrate 32 through a crank351. Then, the tension of the chain 2 is adjusted by virtue of theresilient force of a spring 352 provided in the crank 351. The outergear 36 is juxtaposed with the drive gear 34 and the tension gear 35.Only the inner gear 37 is disposed at a position inside of these gears34, 35 and 36. Accordingly, the chain 2 is adapted to be driven androtated by the drive gear 34 while it passes a gently curved locushaving an inflection point from the inner gear 37 to the outer gear 36.The curvature of the curved locus can be changed by adjusting the lengthof the shaft 371 of the inner gear 37.

In the sheet widening apparatus 1 constructed as mentioned above, theinner gears 37 of the sheet widening apparatus 1 are disposed at thereceiving start point side of the foam sheet S to be supplied from afoaming furnace 6.

Next, a method for widening the foam sheet S using the sheet wideningapparatus 1 will be described in the following.

Initially, before a sheet widening operation by the sheet wideningapparatus 1, the foam sheet S is manufactured.

The foam sheet S is manufactured in such a manner that a thermaldecomposition type foaming agent is blended in a polyolefine resin, andan antioxidant, a crosslinking assistant, a pigment or the like are alsoblended therein, if necessary. The bended product is molten and kneadedat a temperature lower than a temperature at which the foaming agent isdecomposed and the molten and kneaded product is formed in aconfiguration of a sheet by an extruder (a uniaxial extruder, a biaxialextruder, a pressure kneader, etc.) or the like.

Then, the sheet thus extruded is irradiated with an ionizing radiationsuch as α-ray, β-ray, γ-ray, electron beam, etc, and crosslinked. Asmethods for this crosslinking technique may adopt such methods as achemical crosslinking method using an organic peroxide, a method forgrafting or copolymerizing vinyl trimethoxysilane or the like with aresin and closslinking the grafted or copolymerized material with waterand the like may be employed.

The foam sheet S crosslinked as mentioned above is heated at adecomposition temperature or higher of the foaming agent in a foamingfurnace 6 and foamed. Various foaming techniques such as methods forpassing the foam sheet through a preheating area and a foaming area in afoaming furnace using a hot air circulating type vertical foamingfurnace or horizontal foaming furnace have been known (As for a foamingapparatus, refer to Japanese Examined Patent Publication Nos. 42-18832and 48-9955).

The foam sheet S thus obtained immediately after a foam sheet before itis widened is foamed by heating in the foaming furnace 6 undergoes afirst widening step by a vacuum device 7 or the like using vacuumadsorption means. Then, the foam sheet S is supplied to the sheetwidening apparatus 1. The first widening step involves a step forstretching flares (vertical wrinkles) of the foam sheet S immediatelyafter being subjected to heating and foaming processes, so that any kindof widening technique may be employed or it may be omitted.

In the sheet widening apparatus 1, then, both the side parts of the foamsheet S are pierced with the pins 21 of the chains 2 at the positions ofthe inner gears 37 in the sheet receiving start point side. At thistime, in the back surface of the foam sheet S, brush wheels 4 aredisposed and are adapted to support the piercing operations of the pins21. The pins 21 may pierce the foam sheet S so as to pass through thefoam sheet S or to penetrate the foam sheet up to the middle of thesheet. The foam sheet S may or may not be pierced with the pins 21 up totheir roots.

The foam sheet S pierced with the pins 21 are received by a plurality ofsheet receiving rolls 5. At the same time, since the chains 2 pass anddrive the foam sheet along such tracks as to increase the width betweenthe chains 2 from the inner gears 37 to the outer gears 36, the foamsheet S is widened along the tracks of the chains 2.

In this case, the foam sheet is pierced with the pins 21 so that it isnot shifted or displaced in the direction of its width, and therefore,the foam sheet S is substantially widened by the width increased alongthe tracks of the chains 2.

Further, since the widening force of the foam sheet S is distributedover the side parts thereof by piercing them with a number of pins 21,the foam sheet S is effectively prevented from being broken.

The loci or tracks of the chains 2 are gently curved having theinflection points from the inner gears 37 to the outer gears 36, andtherefore, the foam sheet S can be entirely evenly widened while itpasses from the inner gears 37 to the outer gears 36.

Namely, as shown in FIG. 5, the foam sheet S is first gradually widenedin parts L1 of the loci after passing the inner gears 37 curvature ofwhich is loose. In this widening step of sheet, both the side parts S1of the foam sheet S are mainly stretched. Both the side parts S1 arestretched, so that the tensile strength of the side parts S1 can beimproved.

As the foam sheet S advances toward the outer gears 36, the foam sheet Sis abruptly widened at the parts L2, a curvature of which is sharp, onthe loci or tracks of the chains 2. As a result of this wideningoperation, the central part S2 of the foam sheet S is mainly stretched.Since the tensile strength of both the side parts S1 of the foam sheet Shas been already improved at the time of the first widening step, thefoam sheet S is not broken and can bear such an abrupt wideningoperation. Further, since the foam sheet is widened along generallycurved loci, the sheet can be immediately widened without locallyapplying an unreasonable force thereto. Distance L between each of theinner gears 37 and each of the outer gears 36 which is required forwidening the foam sheet can be reduced as shown in FIG. 5. While thefoam sheet S has remaining heat after the foam sheet S is delivered fromthe foaming furnace 6, the foam sheet S can be effectively widenedwithout providing an additional heating source The widened foam sheet Sincludes both the side parts S1 and the central part S2 which areentirely uniformly widened.

In the above described apparatus for widening the foam sheet accordingto the present invention, while an example has been mentioned in whichthe inner gears meshing with a pair of chains and operating as thedriving means at the sheet receiving start points and the inner gears asthe driving means at the sheet receiving end points are provided inparallel to the advancing direction of the sheet, the driving means atthe sheet receiving start points or the driving means at the sheetreceiving end points may be so attached as to be angled relative to theadvancing direction of the sheet or the inclination angle of the drivingmeans may be changed, an example of which will be described hereinbelow.

As shown in FIG. 6, when the inner gears 37 as the driving means at thesheet receiving start points are so attached beforehand as to be angled8 and tapered relative to the advancing direction of the sheet S, theparts L1 of gentle curvature of the loci of the sheet S are lengthened,so that the initial widening operation of the sheet S after passing theinner gears 37 can be completely carried out. If the angle 8 is changed,an initial widening step suitable for the sheet S can be achieved.

As shown in FIG. 7, if each of the inner gears 37 as the driving meansat the sheet receiving star points can be rotated about a supportingpoint P₁ in the direction shown by an arrow X and each of the outergears 36 as the driving means at the sheet receiving end points can berotated about a supporting point P₂ in the direction shown by an arrowY, and the inclination angle of the inner gears 37 and the outer gears36 can be varied relative to the advancing direction of the sheet S, thespace L between each of the inner gears 37 and each of the outer gears36 can be more reduced than in the case of the similarly widened foamsheet S, which contributes for the formation of small size of apparatus.

FIG. 8 shows a sectional view showing another example of the chain ofthe sheet widening apparatus according to the present invention.

In FIG. 8, reference numeral 211 designates a pin inclined and attachedto a base member 25 which is fixed to a receiving part 24 by means of abolt 26 and a nut 27. The receiving part 24 is extended from a linkplate 23 of a link 22 of a chain 2. φ is an inclination angle of the pin211 so attached as to be inclined to the base member 25 of the link 22relative to the link 22.

FIGS. 9 and 10 show the attaching direction of a pin part inclined andattached to the base member 25. FIG. 9 is a plan view showing an exampleof a pin horizontally inclined and attached to the base member andviewed from the side in which the pin is provided. FIG. 10 is a planview showing an example of a pin horizontally and vertically inclinedand attached to the base member and viewed from the side in which thepin is provided.

In FIG. 9, reference numeral 212 designates a pin part horizontally(direction shown by an arrow E) inclined and attached to the basemember. The pin part 212 is horizontally inclined and attached relativeto the base member 25.

In FIG. 10, reference numeral 213 denotes a pin part horizontally(direction shown by an arrow E) and vertically (direction shown by anarrow F) inclined and attached to the base member. The pin part 213 isso attached as to be inclined horizontally and vertically relative tothe base member 25.

In this way, in case where the pins 213 are so attached as to beinclined to the links 22, as shown in FIG. 11, when the pins 213 are tobe pulled out of the foam sheet S, the pins 213 are directed to the samedirection as the advancing direction (direction shown by an arrow B) ofthe foam sheet S and the pins 213 can be easily slipped from the foamsheet S, even if the adavancing direction (direction shown by an arrowA) of the chain 2 is different from the advancing direction (shown bythe arrow B) of the foam sheet S. α is an angle between the advancingdirection of the chain 2 and the advancing direction of the foam sheetS. The above mentioned inclination angle φ corresponds to this angle α.

In case where the pins 213 are not so attached as to be inclinedrelative to the links 22, as shown in FIG. 12, the pins 214 are inclinedby an angle equal to the angle α between the advancing direction(direction shown by an arrow A) of the chain 2 and the advancingdirection (direction shown by an arrow B) of the foam sheet S, if theadvancing direction of the chain 2 is different from the advancingdirection of the foam sheet S. Under this state, when the pins 214 areto be pulled out of the foam sheet S, it is difficult to pull the pins214 out of the foam sheet S. Therefore, such difficulties have beenencountered, as the twining of the foam sheet S on the chains, thebreakage of the sheet or the like.

FIG. 13 is a sectional view showing a still another example of a chainof the sheet widening apparatus according to the present invention.

In FIG. 13, reference numeral 215 designates a pin part attached to abase member 25 fixed to a receiving part 24 by means of a bolt 26 and anut 27. The receiving part 24 is extended from a link plate 23 of a link22 of a chain 2. β is a cone angle of the pin part 213. The cone angle βis larger than the angle α between the advancing direction of the chain2 and the advancing direction of the foam sheet S. The pin part 215 isso formed as to have a thick root and a thin end.

In such a construction, when the pins 215 are to be pulled out of thefoam sheet S, even if the advancing direction of the chains 2 isdifferent from the advancing direction of the foam sheet S, theinclination of the pins 215 relative to the advancing direction of thefoam sheet S is absorbed and therefore, the pins 215 can be easilyslipped from the foam sheet S because the pins 215 respectively have alarge cone angle.

As mentioned before, according to the present invention, a foam sheet isnot broken and completely, entirely and uniformaly stretched by wideningthe foam sheet along the gently curved loci having the inflectionpoints, so that the foam sheet excellent in workability and having nodrawing-down can be obtained.

According to the sheet widening method according to the presentinvention, the foam sheet is not shifted or displaced in the directionof width thereof by piercing both the side parts of the foam sheet withthe pins and the foam sheet can be substantially widened by the space ofwidth between the pins increased. Therefore, the foam sheet can bestrongly widened by widening the space of width between the pins alongthe gently curved loci having the inflection points while the pinspierce both the sides of the foam sheet, and the sufficiently widenedfoam sheet can be obtained.

Further, according to the sheet widening apparatus of the presentinvention, while the foam sheet is pierced with the pins in both thesides thereof at the sheet receiving start points, the foam sheet can besimultaneously widened and supplied to the sheet receiving end points bydriving the chains. Therefore, even if the foam sheet is widenedabruptly to some degree, the foam sheet can be completely widenedwithout releasing the holding state of both the side parts of the foamsheet. The pins holding both the side parts of the foam sheet movedepending on the driving of the chains composed of the links having thepins in question. Thus, even in case where the foam sheet is widenedover multistages, it can be simply and readily widened by changing thedriving tracks or loci of the chains to desired widening conditions.

Still further, according to the sheet widening apparatus of the presentinvention, the driving means provided at the sheet receiving startpoints and meshing with a pair of chains are so attached as to be angledrelative to the advancing direction of the foam sheet and theinclination angle of the driving means can be changed, so that theinitial widening step of the foam sheet can be achieved in accordancewith the characteristics of the foam sheet.

Also, since the driving means at the sheet receiving start points or thedriving means at the sheet receiving end points are so attached as to beangled and inclined relative to the advancing direction of the foamsheet, or the inclination angle of the driving means may be changed, theinclination angle can be changed depending on the characteristics of thefoam sheet, the space between each of the driving means at the sheetreceiving start points and each of the driving means at the sheetreceiving end points, that is, a distance required for widening the foamsheet can be reduced, which contributes for manufacture of the smallsize of apparatus.

Still further, according to the sheet widening apparatus of the presentinvention, since the pins penetrating both the side parts of the foamsheet are so attached as to be inclined relative to the links formingthe chains, the pins can be directed to the same direction as theadvancing direction of the foam sheet at the time of pulling the chainsout of the foam sheet, even when the advancing direction of the chainsis different from that of the foam sheet. Thus, the pins can be smoothlyslipped from the foam sheet. Accordingly, such inconveniences can beprevented, as the winding of foam sheet on the chains, the breakage ofthe sheet or the like due to hard slipping of the pins from the foamsheet.

Furthermore, according to the sheet widening apparatus of the presentinvention, since the pins penetrating the side parts of the foam sheetrespectively have a larger cone angle than the inclination angle of thechains relative to the advancing direction of the foam sheet, theinclination of the pins relative to the advancing direction of the foamsheet is absorbed by virtue of the large cone angle and the pins can bereadily slipped from the foam sheet. As a result, various troubles, suchas the twining of the foam sheet on the chains, the breakage of sheet orthe like because of hard slipping of the pins from the foam sheet, canbe prevented.

What is claimed is:
 1. A method for widening a foam sheet comprising thesteps of:piercing with pins both outer marginal side edges along theadvancing direction of a crosslinked foam sheet obtained immediatelyafter a foam sheet which has not been yet widened is foamed by heating,when said foam sheet is received by a sheet widening apparatus, saidpins being attached to chains which travel along gently curved loci; andwidening the width of the foam sheet between the pins piercing both sideedges of the foam sheet along the gently curved loci while said foamsheet is received from sheet receiving start points to sheet receivingend points with the pins piercing the side edges of the foam sheet suchthat the sheet is stretched first in an inwardly convexed direction andthen in an outwardly convexed direction.
 2. The method as set forth inclaim 1 including the additional step of initially widening the foamsheet in the area immediately adjacent each side edge.
 3. The method asset forth in claim 1 wherein the sheet has portions extending inwardlyfrom each of the side edges and a central portion lying therebetween,said method including the additional steps of initially stretching theside edge portions and thereafter stretching the central portion.
 4. Themethod as set forth in claim 3 wherein the side edge portion stretchingoccurs along a gradual curvature.
 5. The method as set forth in claim 1including the additional steps of increasing the tensile strength of themarginal side edges of the sheet and thereafter stretching the remainingportion of the foam sheet located between such marginal side edges.